π-junction induced by strong inter-pocket transmission at FeTe.55 Se.45(FTS) half-shift domain wall

Recent scanning tunneling spectroscopy(STS) along crystalline domain walls (DWs) associated with a half unit cell shift have revealed sub-gap density of states that are expected to arise from helical Majorana modes. Such propagating Majorana modes have been proposed to exist on the surface state of topological materials similar to FTS along line defects where the superconducting order parameter (OP) is phase shifted by π. Here we propose a mechanism for the emergence of such a π shift in the OP across the half unit-cell shift DW of FTS. Our mechanism relies on the s_± pairing symmetry across Γ and M pockets of FTS. We construct a tight binding model for the FTS mono-layer including the Fe-3d_x²-y² and 3d_xz/yz orbitals and use the KWANT package to solve the scattering problem across the DW. The resultant inter-pocket transmission between Γ and M pockets is found to be typically larger than the intra-pocket transmissions. In parallel, calculation of the Bogoliubov de Gennes(BdG) spectra for the cases of π and 0-junctions at DW shows that the occupied levels have lower energy for the π-junction. We find that the latter calculation often favors a π-shift across the DW when an incoming electron of Γ(M) pocket, due to stronger inter-pocket transmission found in the former calculation, is Andreev reflected as a hole of M(Γ) pocket, which has opposite sign for the OP. We discuss parameter regimes where this mechanism might explain the STS experiments as well as propose to test this explanation by searching for evidence of large inter-pocket scattering.